Why does my dog/cat need to have a blood test before starting heartworm medication?

Your pet should be tested for heartworm infection before he or she is placed on a preventive to avoid any harmful or possibly fatal complications. We also do annual testing for heartworms to ensure your pet’s heartworm prevention is doing it’s job properly (you never know when they may throw up their pill, or spit it out without you seeing it, or just forget to give a dose). For instance, if a heartworm-infected dog is started on a monthly preventive, immature heartworms (called microfilariae) can die suddenly, causing a serious, shock-type reaction. In addition, preventives won’t kill adult heartworms, so an infected dog needs to be started on a treatment plan.